feat(admin): remove all department/campus scoping from frontend

- Delete CampusSwitcher component and useCampus hook
- Delete Departments page and its route
- Remove campus header interceptor from API client
- Remove departmentId from Class interfaces
- Remove campusLocation from student profile
- Remove department permissions from test fixtures
- Remove currentCampusId from test cleanup

TypeScript compiles clean.

apps/admin/src/test/helpers.ts

@@ -47,7 +47,6 @@ export function logout(): void {
   localStorage.removeItem('token');
   localStorage.removeItem('user');
   localStorage.removeItem('permissions');
-  localStorage.removeItem('currentCampusId');
 }
 
 // ── API helpers (authenticated) ─────────────────────────────────────